Circuit for the detection of a defective power supply connection

ABSTRACT

A device for detecting a defective power supply connection in an integrated circuit includes a comparison circuit for comparing voltage levels of an input/output pad of the integrated circuit and an internal power supply line connected to a power supply pad of the integrated circuit. A pull-down or pull-up device is connected between the input/output pad and the internal power supply line.

FIELD OF THE INVENTION

[0001] The present invention relates to detection circuits, and moreparticularly, to a detection circuit for detecting a defective powersupply connection.

BACKGROUND OF THE INVENTION

[0002] Using a smart card as an example, the integrated circuit withinsuch a card receives its power from a coupler or card reader, whichtypically includes Vdd and Gnd. The quality of this card/coupler linkmay be defective, wherein the serial contact resistance values are veryhigh. The quality may be affected by wear and tear or corrosion of thepads of the integrated circuit. Another cause of deterioration in thequality of the card/coupler link is fraud. That is, an ill-intentionedthird party may deliberately damage this link, for example, by puttingadhesive on one or more pads of the card to cause it to operateabnormally.

[0003] When the card/coupler connection is defective with respect to thepower supply pads, the internal power supply of the integrated circuitcan be obtained indirectly by the input/output pads of the integratedcircuit. This is done through the pull-down or pull-up elements (e.g.,resistors and diodes) connected between these pads and an internalsupply voltage. However, this may lead to a significant malfunctioningin the integrated circuit.

[0004] Taking as an example an integrated circuit with two power supplypads for receiving a ground voltage Gnd and a logic supply voltage Vddthrough the coupler, at least one input/output pad with a ground (GND)pull-down element using a resistor will be referenced Rpd. This padreceives a default level from the coupler, which corresponds to ground.This level may change during very short periods of time corresponding toclock pulses of the circuit/coupler link.

[0005] By default, the integrated circuit is in a first mode ofoperation in which it consumes current I1 (for example, in a memory readmode). Upon a command from the coupler, it may pass into a second modeof operation in which it consumes higher current I2 (for example, in amemory write mode).

[0006] An integrated circuit of this type usually comprises a power-onreset device depending on the potential difference between the internallevels of the logic supply voltage and ground in the integrated circuit.This device is active when the circuit is powered on and off. Inparticular, this device is activated whenever the potential differencebetween the internal levels of the logic supply voltage and ground fallsbelow a specified threshold. This threshold shall be referenced Vpor. Itis assumed in the example that the threshold Vpor is set at 4 volts fora five-volt logic power supply voltage Vdd.

[0007] Assuming that the power supply pad associated with ground GNDshows a very high series contact resistance, ground Gnd is not properlytransmitted within the integrated circuit by the pad. The input/outputpad which is grounded brings the internal ground line to a voltage levelVA by the associated pull-down device.

[0008] If the integrated circuit is in the first mode of operation, thisinternal ground level VA obtained by the pull-down resistor Rpd is equalto Rpd*I1. This means that the integrated circuit could operate properlyin this first mode of operation provided that the internal ground levelV1 is such that the ground potential difference between the internallevel Vddint of the logic supply voltage Vdd and this internal level ofground remains higher than the reset threshold Vpor. This is written asfollows:

Vddint−Va>Vpor; that is

Vddint−Rpd*I1>Vpor.

[0009] If the integrated circuit goes into the second mode of operation,the internal level Va of ground goes to a higher level (I2>I1) equal toRpd*I2. It is then possible to have a situation where the potentialdifference between the internal levels of the logic supply voltage Vddand ground Gnd fall below the reset threshold Vpor of the integratedcircuit. This will deactivate the integrated circuit. Since this changein situation is not immediate, it may happen that a portion of thecommand related to the second mode of operation (writing in memory) isexecuted. This is even more disadvantageous.

SUMMARY OF THE INVENTION

[0010] In view of the foregoing invention, an object of the invention todetect a defective connection of an integrated circuit with a powersupply.

[0011] Another object of the invention is to detect high values ofseries contact resistances at the power supply pads of an integratedcircuit.

[0012] Yet another object of the invention is to detect when the powersupply for an integrated circuit is not obtained by the input/output padpull-down or pull-up devices of the integrated circuit.

[0013] These and other objects, advantages and features of the inventionare provided by at least one voltage level comparison device thatcompares voltage levels between an internal power supply line of theintegrated circuit and an input/output pad of the integrated circuit.The integrated circuit comprises a pull-down or pull-up device connectedbetween the input/output pad and the internal power supply line.

[0014] If the voltage level at the internal supply line is dictated bythe pull-down or pull-up device, this level may be greater than or lowerthan the voltage level of the pad. Depending on whether this differenceis above or below a predetermined detection threshold, the supply isdetermined to be a good connection or a bad connection. Should thesupply be detected as being a bad connection, the integrated circuitwill inhibit its own operation. The detection device according to theinvention then acts as a safety device, preventing any malfunctioning inthe integrated circuit that might be caused by a defective power supplyconnection.

[0015] The detection device according to the invention preferablycomprises a comparison circuit associated with each internal supplylevel. If, for a given supply voltage level, there are severalinput/output pads, with a pull-down or pull-up device at this voltagelevel, then a comparison device is preferably provided for each of thesepads. A detection device according to the invention is especiallyrecommended for actively combating attempts at fraud designed to giverise to the abnormal operation of the integrated circuit.

[0016] The device can also be applied in cases of normal use of theintegrated circuit to detect wear and tear or corrosion of the supplypads. In this type of application, the detection device will preferablybe used in combination with protection circuits that may exist at theinput/output pads themselves. These input/output pads may themselves beworn out or corroded due to the normal use of the integrated circuit.Hence, the power level transmitted by these pads may be degraded and theoutput results of the detection device according to the invention mustthen be considered in combination with other safety information.

[0017] As claimed, the invention therefore relates to a device for thedetection of a defective power supply connection in an integratedcircuit. According to the invention, in an integrated circuit comprisingat least one power supply pad for applying an external power supply toan internal power supply line of the integrated circuit and at least oneinput/output pad with which there is associated a pull-down or pull-updevice connected between the pad and the internal supply line, thedetection device comprises a circuit for the comparison of the voltagelevels between the pad and the internal supply line. The signaldelivered by the detection device may be used to inhibit the internaloperation of the integrated circuit.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Other features and advantages of the invention are given in thefollowing description, by way of a non-restrictive indication, withreference to the appended drawings of which:

[0019]FIG. 1 is a general block diagram of the detection device in anintegrated circuit according to the invention; and

[0020]FIG. 2 is a detailed schematic diagram of an exemplary embodimentof the detection device according to the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0021]FIG. 1 is a block diagram of an integrated circuit CI havingconnection pads for receiving signals from a coupler CP. The integratedcircuit comprises a detection device 2 according to the invention.

[0022] The integrated circuit CI has a main circuit 1, and the detectioncircuit 2 for detecting defective power supply connections and defectiveconnection pads. A first supply connection pad PAD-V1 is connected to afirst internal logic supply line BV1 and receives an external supplyvoltage V1 ext from the coupler CP. A second connection pad PAD-V2 isconnected to a second internal logic supply line BV2 and receives anexternal supply voltage V2 ext from the coupler CP.

[0023] The values are chosen such that V1 ext>V2 ext. These logic supplyvoltages may, for example, correspond respectively to a positive logicsupply voltage Vdd, for example, of about 3.3 volts, and to ground Gnd.However, the invention is not limited to any particular logic supplyvoltage levels. When the connections are high-quality connections, aninternal supply level is received on the internal power supply lines,with V1 int and V2 int respectively corresponding to the levels of theexternal supply voltages applied to the power supply pads, namely V1 extand V2 ext.

[0024] The integrated circuit CI also comprises input/output pads with apull-down or pull-up device at one of the internal supply voltagelevels. In the example, a first input/output pad PAD-A is associatedwith a pull-down or pull-up device 4 connected between this pad and thesecond internal supply line BV2. The coupler CP sets this first padPAD-A by default at the level of the second external supply voltage V2ext. A second input/output pad PAD-B is associated with a pull-down orpull-up device 5 connected between this pad and the first internalsupply line BV1. The coupler CP sets this second pad by default at thelevel of the first external supply voltage V1 ext.

[0025] In the example shown, the detection device 2 has a detectioncircuit associated with each internal supply line. A detection circuit20 has inputs connected to the first internal supply line BV1 and thesecond input/output pad PAD-B. The detection circuit 20 provides adetection signal Det1 at its output as a function of the internal supplyvoltage level V1 int on the internal supply line BV1 and of the voltagelevel on the input/output pad PAD-B. If the connection to the supply padPAD-V1 is defective, the voltage level on the internal supply line BV1is dictated by the pull-down or pull-up device 5 associated with theinput/output pad PAD-B.

[0026] In this case, the voltage level V1 int on the internal powersupply line BV1 is lower than the real level of the external supplyvoltage V1 ext, which is the voltage obtained on the pad PAD-B. If, onthe contrary, the connection is good, then the level is substantiallythe same on the internal power supply line BV1 and on the input/outputpad PAD-B.

[0027] Similarly, inputs of a detection circuit 21 are connected to thesecond supply line and to the input/output pad PAD-A. At its output,this detection circuit 21 provides a detection signal Det2, as afunction of the levels at the two inputs. If the connection to thesupply pad PAD-V2 is defective, the voltage level V2 int on the internalpower supply line BV2 is imposed by the pull-down or pull-up device 4associated with the input/output pad PAD-A.

[0028] In this case, the voltage level V2 int on the internal powersupply line BV2 is higher than the real level of the external supplyvoltage V2 ext, which is the voltage level obtained on the pad PAD-A.If, on the contrary, the connection is good, then the level issubstantially the same on the internal supply line and on theinput/output pad.

[0029] In the example, an output signal DET of the detection device 2 isactivated when at least one of the detection signals Det1 or Det2 isactivated. This indicates a defective connection on at least one of thelogic supply pads. The signal DET is sent to the main circuit 1 of theintegrated circuit CI, which operates as an alarm signal. In particular,the main circuit 1 can activate safety devices (not shown) that willblock its operation.

[0030] An integrated circuit may comprise several input/output pads witha pull-down or pull-up device at one of the internal supply voltagelevels. In this case, it is possible to provide for one detectioncircuit per existing input/output pad to provide similar protection ofthe integrated circuit CI.

[0031]FIG. 2 shows an exemplary embodiment of comparison circuits usedin a protection device according to the invention. In this example, eachcircuit comprises at least two inverters. A first inverter compares thevoltage levels of the internal power supply line and of the input/outputpad, and a second inverter reshapes the signal obtained and provides thedetection signal at its output.

[0032]FIG. 2 shows the comparison circuits 20 and 21 for the twointernal power supply lines BV1 and BV2. In the case of the internalpower supply line BV1, the associated comparison circuit 20 is used todetect the fact that the voltage level V1 int of this line is lower thanthe voltage level of the input/output pad PAD-B by about one transistorthreshold level. In the case of the internal supply line BV2, theassociated comparison circuit 21 is used to detect the fact that thevoltage level V2 int of this line is lower than the voltage level of theinput/output pad PAD-A by about one transistor threshold level.

[0033] The comparison circuit 20 associated with the internal supplyline BV1 thus comprises a first inverter 200 and a second inverter 201.The first inverter 200 is supplied between the input/output pad PAD-Band the internal power supply line BV2. The input of this inverter isconnected to the internal power supply line BV1.

[0034] As long as the level of the input voltage, namely the internalsupply level V1 int, remains higher than or equal to the voltage levelof the pad PAD-B applied as a positive supply voltage of the inverter,the output of the inverter remains at the low level. If the level V1 intof the input voltage becomes lower than the voltage level of the padPAD-B, the output of the inverter switches over to the high level. Thisprovides for the detection according to the invention.

[0035] In practice, it is sufficient that the level V1 int should becomesmaller than the level of the pad PAD-B by one transistor threshold.Preferably, the transistors of the inverter 200 of the comparisoncircuit are chosen so that they have a threshold voltage that is as lowas possible. The second inverter 201 for its part is supplied normally,by the two internal supply lines BV1 and BV2 and enable the reshaping ofthe output signal of the first inverter.

[0036] Similarly, the comparison circuit 21 associated with the internalsupply line BV2 has a first inverter 210 and a second inverter 211. Thefirst inverter 210 is powered between the internal supply line BV1 andthe input/output pad PAD-A. The input of this inverter is connected tothe internal supply line BV2.

[0037] As long as the level of the input voltage, namely the internalsupply level V2 int remains lower than or equal to the level of thevoltage of the pad PAD-A, applied as a negative supply voltage of theinverter, the output of the inverter remains at the high level. If thelevel V2 int of the input voltage becomes higher than the level of thevoltage of the pad PAD-A, the output of the inverter switches over tothe low level, thus providing for the detection according to theinvention.

[0038] In practice, it is sufficient that the level V2 int should becomehigher than the voltage level of the pad PAD-A by one transistorthreshold. Preferably, the transistor of the inverter 210 of thecomparison circuit is chosen so that it has a threshold voltage that isas low as possible. The second inverter 211 for its part is suppliednormally by the two internal supply lines BV1 and BV2 and is used toreshape the output signal of the first inverter. It will be noted thatthe output logic of the two comparison circuits 20 and 21 is inverted.

[0039] Other embodiments are possible, for example, using differentialamplifiers. The embodiment using simple inverters has the advantage ofbeing a low-cost embodiment in terms of space requirements on theintegrated circuit and in terms of cost. In a practical example, theinput/output pads PAD-A and PAD-B used for internal power supply in theevent of a defective connection at the power supply pads may be pads toreceive control signals by which the integrated circuit is placed in aparticular mode of operation. These pads are then, by default, atvoltage levels corresponding to a default mode of operation.

[0040] The change in the mode of operation of the integrated circuit iscontrolled by imposing a different logic voltage level on either of thepads for the duration of an edge or a clock pulse. In such a case, thedetection according to the invention is permanent or semi-permanent.Such an operation corresponds especially to the mode of operation ofintegrated circuits designed for smart card type applications.

[0041] However, the invention cannot be applied exclusively to this typeof integrated circuit. It can be applied to any integrated circuitcomprising at least one input/output pad connected to an internal powersupply line by a pull-down or pull-up device, corresponding to thedefault voltage level on this pad.

[0042] In particular, there are other input/output pads associated withpull-down or pull-up devices at one of the internal voltage supplylevels, for example, the clock signal transmission pad which is commonlyassociated with a ground pull-down device or data input/output padswhich are commonly associated with a pull-up device for pulling up tothe positive logic supply voltage Vdd. In these examples, the comparisoncircuit associated with such a pad is active only when the level of thepad is equal to the pull-up or pull-down level.

That which is claimed is:
 1. A device for the detection of a defectivepower supply connection in an integrated circuit, comprising at leastone power supply pad to apply an external power supply to an internalpower supply line of the integrated circuit and at least oneinput/output pad with which there is associated a pull-down or pull-updevice connected between said pad and said internal power supply line,wherein the detection device comprises a circuit for the comparison ofthe voltage levels between said pad and said internal power supply line.2. A detection device according to claim 1, comprising at least onecomparison circuit per supply pad of the integrated circuit.
 3. A deviceaccording to claim 1 or 2, comprising a comparison circuit between thevoltage level of an internal power supply line and each input/output padwith which there is associated a pull-down or pull-up device connectedbetween this pad and said line.
 4. A detection device according to anyof the above claims, wherein a comparison circuit for comparing thevoltage levels of an internal power supply line and an input/output padcomprises an inverter supplied by said pad and connected at input tosaid internal power supply line.
 5. A detection device according toclaim 4, wherein said inverter is formed by transistors having athreshold that is the lowest possible threshold.
 6. A detection deviceaccording to claim 4 or 5, wherein said comparison circuit comprisesanother inverter series-connected with the first one, which gives adetection signal at output.
 7. A detection device according to any ofthe above claims, giving an output detection signal applied as a signalto turn off the integrated circuit.
 8. An integrated circuit comprisinga detection device according to any of the above claims.